1. Field of the Invention
The present invention relates to the use of supercritical fluids to clean substrates, and, more particularly, to a process and apparatus employing a liquefiable gas, such as liquid carbon dioxide, in combination with ultrasonic cavitation to provide high cleaning efficiency for general degreasing and particulate removal without the need for expensive high pressure equipment.
2. Description of Related Art
Ultrasonic cleaning has been utilized by industry for a number of years. In the conventional processes, the sonicating media are organic solvents, water, or aqueous solutions, and ultrasonic energy is applied to the media to promote cavitation, i.e., the formation of bubbles and their subsequent collapse. Although usually quite adequate for the removal of undesired contamination, both types of solvents have disadvantages. Many substrates require a rigorous drying process following exposure to an aqueous medium, and this is often a time-consuming thermal excursion. The use of organic solvents as sonicating media presents the problem of chemical disposal and is subject to strict regulatory controls. An additional disadvantage relates to handling of the removed contaminant(s), whether organic or particulate. When the contaminant is a controlled material, such as a radioactive particle, once in solution or suspension, its volume is substantially increased, and this presents an additional pretreatment/disposal problem.
In these conventional ultrasonic cleaning processes, transducers are often used to produce the sonic energy. In other processes, a cavitation nozzle may be used. For example, U.S. Pat. No. 4,906,387, issued Mar. 6, 1990, to J. Pisani for "Method for Removing Oxidizable Contaminants in Cooling Water Used in Conjunction with a Cooling Tower" and U.S. Pat. No. 4,990,260, issued Feb. 5, 1991, to J. Pisani for "Method and Apparatus for Removing Oxidizable Contaminants in Water to Achieve High Purity Water for Industrial Use" disclose methods for removing contaminants from water by inducing cavitation in the water to cause the water to dissociate to produce hydroxyl free-radicals which act as oxidizing agents. In the processes of Pisani, ultraviolet radiation is used in combination with cavitation to continue the oxidation process which was initiated by the hydroxyl free-radicals. The cavitation in the Pisani processes is produced by a "critical flow" nozzle.
Another type of cleaning process, utilizing phase shifting of dense phase gases, has been disclosed and claimed in U.S. Pat. No. 5,013,366, issued to D.P. Jackson et al and assigned to the same assignee as the present application. The process employs a dense phase gas at or above the critical pressure. The phase of the dense phase gas is then shifted between the liquid state and the supercritical state by varying the temperature of the dense fluid in a series of steps between temperatures above and below the critical temperature of the dense fluid, while maintaining the pressure above the critical value. Examples of fluids include (1) hydrocarbons, such as methane, ethane, propane, butane, pentane, hexane, ethylene, and propylene; (2) halogenated hydrocarbons, such as tetrafluoromethane, chlorodifluoromethane, and perfluoropropane; (3) inorganics, such as carbon dioxide, ammonia, helium, krypton, argon, sulfur hexafluoride, and nitrous oxide; and (4) mixtures thereof. In alternative embodiments, the dense phase gas may be exposed to ultraviolet (UV) radiation during the cleaning process or ultrasonic energy may be applied during the cleaning process to agitate the dense phase gas and the substrate surface.
In yet another approach to cleaning, components are placed in a cleaning chamber that is maintained at a pressure above ambient atmospheric pressure and a spray of liquid solvent, such as liquid carbon dioxide, under high pressure is directed onto the components so as to dislodge any contaminant particles therefrom. Such an approach is disclosed, for example, in U.S. Pat. Nos. 4,832,753 and 4,936,922 by R.L. Cherry et al. The spray of tiny solvent droplets act as "fluid hammers" to knock very small, submicrometer particles off of the components to be cleaned, dispersing the particles into the chamber where they are carried away by a stream of clean, dry air flowing over the components and through the chamber.
Systems based on supercritical fluid cleaning technology, such as the SUPERSCRUB.TM. precision cleaning equipment (a trademark of Hughes Aircraft Company), typically include a pressure vessel, a fluid pump, a fluid reservoir, a separator and condenser system, and various valves, transducers, and temperature sensors. The pressure vessel that is employed is capable of containing pressures up to 5,000 psi (351.5 kg/cm.sup.2) and temperatures up to about 100.degree. C. This technology provides the conditions required to exceed the critical points of most candidate supercritical fluids, such as nitrogen, oxygen, argon, helium, methane, propane, carbon dioxide, and nitrous oxide. Systems of this nature are expensive. The cost of such systems is well-justified for high precision cleaning; however, for many particulate and organic contaminants, the cleaning process criteria may often be met without using these fluids in their supercritical state.
Thus, a need exists for a system that provides for simplified and reliable performance for small scale and "low end" cleaning applications.